Penrith, just 50 kilometres inland, may still reach 39 degrees according to the bureau, well short of the blistering 45 degrees earlier predicted by the bureau. Regions in the far north-west of the state, such as Bourke, are forecast to endure 47 degrees - with the mercury climbing even higher in parts of north-east South Australia.

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"The cool change has arrived earlier than the models were predicting," said Rob Sharpe, a meteorologist with Weatherzone.

The cooler conditions reached Sydney Airport at just after 11, and are bringing the potential for storms, such as to the Windsor region, Mr Sharpe said.

Beyond the coastal regions, the heat will make it another tough day for firefighters. Authorities have declared a total fire ban across the state for a second day in a row.

The Rural Fire Service earlier today said 16 fires are yet to be contained.

Today’s westerly winds are expected to add to the bushfire risk, with three fires posing the greatest concern - the Deans Gap fire in the Shoalhaven, the Cobbler Road fire near Yass, and the Yarrabin fire in the Cooma-Monaro region.

Central Australia’s giant heat cell is showing little sign of breaking up, with only ‘‘some indications’’ that it may start to cool off in ‘‘about a week’s time’’, Mr Casper-Richardson said.

The hottest region in the country on Saturday is likely to be the north-east of South Australia, with Moomba headed for as high as 49 degrees, with a similar peak for Sunday. ‘‘There’s an outside chance of 50,’’ Mr Casper-Richardson said.

Moomba was at 47.9 degrees by the early afternoon, local time, while Bourke had hit 46.4, placing the 49.7 degree record for the town and the state within range.

Tropical Cyclone Narelle, meanwhile, continues to skirt the north-western coast of WA, and will gradually weaken, he said. Wind gusts at Karratha have so far reached a maximum of 83 kilometres an hour.

While the cyclone is bringing cloud and some moisture over the continent, it ‘‘will take more to disperse the heat over the inland,’’ Mr Cooper-Richardson said.

So far, most of the cool changes coming over southern regions have brought little rain, with the heat dominating central Australia contributing to drier conditions in the south-east, he said.

This month's heatwave has produced seven of the top 20 hottest days - based on average maximum temperatures - in the past century of records.

Tuesday saw the highest mean temperature - averaging out maximums and minimum - across the nation. Set at 32.32 degrees, the mercury was almost half a degree higher than records that had stood since the previous great heatwave in 1972-73.